On Wed, Jul 18, 2018 at 1:52 AM, Paul Burton paul.burton@mips.com wrote:
Hi Huacai,
On Fri, Jul 13, 2018 at 03:37:57PM +0800, Huacai Chen wrote:
Linux expects that if a CPU modifies a memory location, then that modification will eventually become visible to other CPUs in the system.
On Loongson-3 processor with SFB (Store Fill Buffer), loads may be prioritised over stores so it is possible for a store operation to be postponed if a polling loop immediately follows it. If the variable being polled indirectly depends on the outstanding store [for example, another CPU may be polling the variable that is pending modification] then there is the potential for deadlock if interrupts are disabled. This deadlock occurs in qspinlock code.
This patch changes the definition of cpu_relax() to smp_mb() for Loongson-3, forcing a flushing of the SFB on SMP systems before the next load takes place. If the Kernel is not compiled for SMP support, this will expand to a barrier() as before.
References: 534be1d5a2da940 (ARM: 6194/1: change definition of cpu_relax() for ARM11MPCore) Cc: stable@vger.kernel.org Signed-off-by: Huacai Chen chenhc@lemote.com
arch/mips/include/asm/processor.h | 10 ++++++++++ 1 file changed, 10 insertions(+)
diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h index af34afb..a8c4a3a 100644 --- a/arch/mips/include/asm/processor.h +++ b/arch/mips/include/asm/processor.h @@ -386,7 +386,17 @@ unsigned long get_wchan(struct task_struct *p); #define KSTK_ESP(tsk) (task_pt_regs(tsk)->regs[29]) #define KSTK_STATUS(tsk) (task_pt_regs(tsk)->cp0_status)
+#ifdef CONFIG_CPU_LOONGSON3 +/*
- Loongson-3's SFB (Store-Fill-Buffer) may get starved when stuck in a read
- loop. Since spin loops of any kind should have a cpu_relax() in them, force
- a Store-Fill-Buffer flush from cpu_relax() such that any pending writes will
- become available as expected.
- */
I think "may starve writes" or "may queue writes indefinitely" would be clearer than "may get starved".
Need I change the comment and resend? Or you change the comment and get merged?
Huacai
+#define cpu_relax() smp_mb() +#else #define cpu_relax() barrier() +#endif
/*
- Return_address is a replacement for __builtin_return_address(count)
-- 2.7.0
Apart from the comment above though this looks better to me.
Re-copying the LKMM maintainers - are you happy(ish) with this?
Thanks, Paul